This invention relates in general to electrosatatographic imaging and, more particularly, to fusing stations and rollers useful for color imaging having a stiffening layer included within an internally-heated, compliant toner fuser roller used with a compliant pressure roller.
In electrostatographic imaging and recording processes such as electrophotographic reproduction, an electrostatic latent image is formed on a primary image-forming member such as a photoconductive surface and is developed with a thermoplastic toner powder to form a toner image. The toner image is thereafter transferred to a receiver, e.g., a sheet of paper or plastic, and the toner image is subsequently fused to the receiver in a fusing station using heat or pressure, or both heat and pressure. The fuser member can be a roller, belt, or any surface having a suitable shape for fixing thermoplastic toner powder to the receiver. The fusing step in a roller fuser commonly consists of passing the toned receiver between a pair of engaged rollers that produce an area of pressure contact known as a fusing nip. In order to form said nip, at least one of the rollers typically has a compliant or conformable layer on its surface. Heat is transferred from at least one of the rollers to the toner in the fusing nip, causing the toner to partially melt and attach to the receiver. In the case where the fuser member is a heated roller, a resilient compliant layer having a smooth surface is typically used which is bonded either directly or indirectly to the core of the roller. Where the fuser member is in the form of a belt, e.g., a flexible endless belt that passes around the heated roller, it typically has a smooth, hardened outer surface.
Most roller fusers, known as simplex fusers, attach toner to only one side of the receiver at a time. In this type of fuser, the roller that contacts the unfused toner is commonly known as the fuser roller and is usually the heated roller. The roller that contacts the other side of the receiver is known as the pressure roller and is usually unheated. Either or both rollers can have a compliant layer on or near the surface. In most fusing stations having a fuser roller and an engaged pressure roller, it is common for only one of the two rollers to be driven rotatably by an external source. The other roller is then driven rotatably by frictional contact.
In a duplex fusing station, which is less common, two toner images are simultaneously attached, one to each side of a receiver passing through a fusing nip. In such a duplex fusing station there is no real distinction between fuser roller and pressure roller, both rollers performing similar functions, i.e., providing heat and pressure.
Two basic types of simplex heated roller fusers have evolved. One uses a conformable, or compliant, pressure roller to form the fusing nip against a hard fuser roller, such as in a Docutech 135 machine made by the Xerox Corporation. The other uses a compliant fuser roller to form the nip against a hard or relatively non-conformable pressure roller, such as in a Digimaster 9110 machine made by Heidelberg Digital LLC. A fuser roller designated herein as compliant, typically includes a conformable layer having a thickness greater than about 2 mm and in some cases exceeding 25 mm. A fuser roller designated herein as hard, includes a rigid cylinder which can have a relatively thin polymeric or conformable elastomeric coating, typically less than about 1.25 mm thick. A fuser roller used in conjunction with a hard pressure roller tends to provide easier release of a receiver from the heated fuser roller, because the distorted shape of the compliant surface in the nip tends to bend the receiver towards the relatively non-conformable pressure roller and away from the much more conformable fuser roller.
A conventional toner fuser roller includes a cylindrical core member, often metallic such as aluminum, coated with one or more synthetic layers which typically include polymeric materials made from elastomers.
The most common type of fuser roller is internally heated, i.e., a source of heat is provided within the roller for fusing. Such a fuser roller normally has a hollow core, inside of which is located a heating source, usually a lamp. Surrounding the core is an elastomeric layer through which heat is conducted from the core to the surface, and the elastomeric layer typically contains fillers for enhanced thermal conductivity. A different kind of fuser roller which is internally heated near its surface is disclosed by Lee et al. in U.S. Pat. No. 4,791,275, which describes a fuser roller including two polyimide Kapton(copyright) sheets (sold by DuPont and Nemours) having a flexible ohmic heating element disposed between the sheets. The polyimide sheets surround a conformable polyirmide foam layer attached to a core member. According to J. H. DuBois and F. W. John, Eds., in Plastics, 5th Edition, Van Nostrand and Rheinhold, 1974, polyimide at room temperature is fairly stiff with a Young""s modulus of about 3.5 GPa-5.5 GPa (1 GPa=1 GigaPascal=109 Newton/m2), but the Young""s modulus of the polyimide sheets can be expected to be considerably lower at the stated high operational fusing temperature of the roller of at least 450xc2x0 F.
An externally heated fuser roller is used, for example, in an Image Source 120 copier, marketed by Eastman Kodak Company, and is heated by surface contact between the fuser roller and one or more heating rollers. Externally heated fuser rollers are also disclosed by O""Leary, U.S. Pat. No. 5,450,183, and by Derimiggio et al., U.S. Pat. No. 4,984,027.
A compliant fuser roller can include a conformable layer of any useful material, such as for example a substantially incompressible elastomer, i.e., having a Poisson""s ratio approaching 0.5. A substantially incompressible conformable layer including a poly(dimethyl siloxane) elastomer has been disclosed by Chen et al. in U.S. patent application Ser. No. 08/879,896, which is hereby incorporated by reference. Alternatively, the conformable layer can include a relatively compressible foam having a value of Poisson""s ratio much lower than 0.5. A conformable polyimide foam layer is disclosed by Lee in U.S. Pat. No. 4,791,275, and a lithographic printing blanket is disclosed by Goosen et al. in U.S. Pat. No. 3,983,287, including a conformable layer containing a vast number of frangible rigid-walled tiny bubbles which are mechanically ruptured to roduce a closed cell foam having a smooth surface.
Receivers remove the majority of heat during fusing. Since receivers can have a narrower length measured parallel to the fuser roller axis than the fuser roller length, heat can be removed differentially, causing areas of higher temperature or lower temperature along the fuser roller surface parallel to the roller axis. Higher or lower temperatures can cause excessive toner offset in roller fusers. However, if differential heat can be transferred axially along the fuser roller by layers within the fuser roller having high thermal conductivity, the effect of differential heating can be reduced.
Improved heat transfer from the core to the surface of an internally heated roller fuser will reduce the temperature of the core as well as that of mounting hardware and bearings that are attached to the core. Similarly, improved heat transfer to the surface of an externally heated fuser roller from external heating rollers will reduce the temperature of the external heating rollers as well as the mounting hardware and bearings attached to the external heating rollers.
When the fuser and pressure rollers of a simplex fusing station are pressed against each other, and the conformable layer is deflected to form the fusing nip, the thickness of the conformable layer is reduced inside the nip. When the conformable layer is substantially incompressible, the average speed of the conformable layer through the fusing nip must be greater than that of other parts of the conformable layer that are well away from the nip, because the volume flow rate of the elastomer is constant around the roller. This results in a surface speed of the conformable roller inside the nip which is faster than far away from the nip. When, for example, the conformable roller is a driving roller frictionally rotating a relatively non-conformable pressure roller, the pressure roller will rotate faster than if the fuser roller had been non-compliant, a phenomenon known as xe2x80x9coverdrivexe2x80x9d. Overdrive can be expressed quantitatively as a peripheral speed ratio, measured as the ratio of the peripheral surface speeds far away from the nip.
A substantially incompressible elastomer that is displaced in the fusing nip results in an extra thickness of the conformable layer adjacent to either side of the fusing nip, i.e., pre-nip and post-nip bulges. Again, since the elastomer is substantially incompressible, the average speed of the conformable layer in these bulges is less than that of the other parts of the conformable layer that are well away from the nip. The highest pressure in the nip will be obtained at the center of the nip (at the intersection of the joined surfaces and an imaginary line between the centers of the two rollers). Since one roller drives the other, the surface velocities of the rollers should be close to equal at the point of maximum pressure, at the center of the nip. In view of these facts, it can be understood that in general there will be locations in the contact zone of the nip where the surface velocities of the two rollers differ, i.e., there will be slippage. This slippage, which can be substantial just after entry and just before exit of the nip, is a cause of wear which shortens roller life.
A potentially serious problem for fusing arising from the presence of overdrive is xe2x80x9cdifferential overdrivexe2x80x9d, associated for example with tolerance errors in mounting the rollers forming the fusing nip, or with roller runout. Runout can have many causes, e.g., fluctuations in layer thicknesses along the length of a roller, variations in the dimensions of a core member, an acentric roller axis, and so forth. It will be evident that differential overdrive can result in localized differential slippages along the length of a fusing nip, inasmuch as the local effective speed ratio would otherwise tend to fluctuate or change with time along the length of the nip, causing some portions of the driven roller to try to lag and other portions to try to move faster than the average driven speed. Differential overdrive can have serious consequences for fusing, including the formation of large scale image defects and wrinkling of a receiver.
All rollers suffer from surface wear, especially where the edges of receivers contact the rollers. Since relative motion due to slippage between rollers increases wear, the changes in velocity of the surface of a conformable roller, as it travels into, through, and out of a fusing nip formed with a relatively non-conformable roller, should increase the wear rate of the conformable roller, especially if the conformable roller is the heated fusing member, bearing in mind that a fuser roller typically faces a relatively rough and abrasive paper surface in the nip. Moreover, since the material on the conformable roller is stretched and relaxed each time it passes through the fusing nip, this flexure can result in fatigue aging and wear, including failure of the roller due to splitting or cracking of the compliant material, or even delamination.
To obtain high quality electrophotographic copier/printer image quality, image defects must be reduced. One type of defect is produced by smearing of image dots or other small-scale image features in the fusing nip. Relative motions associated with overdrive and resulting in localized slippage between rollers in a fusing nip can cause softened toner particles to smear parallel to the direction of motion, resulting for example in elongated dots.
Some roller fusers rely on film splitting of low viscosity oil to enable release of the toner and (hence) receiver from the fuser roller. Relative motion in the fusing nip can disadvantageously disrupt the oil film.
The Kodak Ektaprint 3100 Copier/Duplicator and the Kodak 1392 Printer both have a fusing station using a compliant fuser roller having 4 cylindrical layers including a buried fluoroelastomeric layer, plus a relatively non-compliant pressure roller. Attached to a cylindrical aluminum core of the fuser roller is a filled silicone rubber conformable layer approximately 2.3 mm thick. Attached to the conformable layer is a fluorelastomeric layer 0.025 mm thick, surrounded by a surface layer approximately 0.23 mm thick made of the same filled silicone rubber as the conformable layer. The fluoroelastomeric layer prevents degradative absorption of release oil from the surface layer into the conformable layer. The surface velocity of the conformable fuser roller far away from the nip is less than that of the relatively non-conformable pressure roller, which is a measure of overdrive. The amount of overdrive is not noticeably different from that produced by a similar compliant roller which lacks the fluoroelastomeric layer.
A toner fuser roller commonly includes a hollow cylindrical core, often metallic, that typically has a heating source in its interior. A resilient base cushion layer, which can contain filler particles to improve mechanical strength and/or thermal conductivity, is formed on the surface of the core, which can advantageously be coated with a primer to improve adhesion of the resilient layer. Roller cushion layers are commonly made of silicone rubbers or silicone polymers such as, for example, poly(dimethylsiloxane) (PDMS) polymers of low surface energy, which minimize adherence of toner to the roller.
Frequently, release oils composed of, for example, poly(dimethylsiloxanes) are also applied to the fuser roller surface to prevent the toner from adhering to the roller. Such release oils (commonly referred to as fuser oils) can interact with the PDMS in the resilient layer upon repeated use, which in time causes swelling, softening, and degradation of the roller. To prevent these deleterious effects caused by release oil, a thin barrier layer of, for example, a cured polyfluorocarbon, is formed on the cushion layer.
Electrophotography can be used to create high quality multicolor toner images when the toner particles are small, that is, diameters less than 10 micrometers, and the receivers, typically papers, are smooth. A typical method of making a multicolor toner image involves trichromatic color synthesis by subtractive color formation. In such synthesis, successive imagewise electrostatic images, each representing a different color, are formed on a photoconductive element, and each image is developed with a toner of a different color. Typically, the colors correspond to each of the three subtractive primary colors (cyan, magenta and yellow) and, optionally, black. The imagewise electrostatic images for each of the colors can be made successively on the photoconductive element by using filters to produce color separations corresponding to the colors in the image. Following development of the color separations, each developed separation image can be transferred from the photoconductive element successively in registration with the other color toner images to an intermediate transfer member. All the color toner images can then be transferred in one step from the intermediate transfer member to a receiver, where they are fixed or fused to produce a multicolor permanent image. Alternatively, an electrophotographic apparatus including a series of tandem modules can be employed, such as disclosed in U.S. patent application Ser. No. 09/199,896, filed in the names of Herrick et al., wherein color separation images are formed in each of four color modules and transferred in register to a receiver member as the receiver member is moved through the apparatus while supported on a transport web.
To rival the photographic quality produced using silver halide technology, it is desirable that these multicolor toner images have high gloss. To this end, it is desirable to provide a very smooth fusing member contacting the toner particles in the fusing station.
In the fusing of the toner image to the receiver, the area of contact of a conformable fuser roller with the toner-bearing surface of a receiver sheet as it passes through the fusing nip is determined by the amount pressure exerted by the pressure roller and by the characteristics of the resilient cushion layer. The extent of the contact area helps establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser roller.
A fuser module is disclosed by M. E. Beard et al., in U.S. Pat. No. 6,016,409, which includes an electronically-readable memory permanently associated with the module, whereby the control system of the printing apparatus reads out codes from the electronically readable memory at install to obtain parameters for operating the module, such as maximum web use, voltage and temperature requirements, and thermistor calibration parameters.
A well known problem in fusing is that paper receiver sheets can not be perfectly rectangular, as a result of humidity-induced swelling. After manufacture, paper sheets are typically stacked and conditioned in a humidity controlled environment. During this time, moisture partially penetrates the paper through the edges of the sheets. For typical commercial paper used in electrophotographic machines, moisture penetration is much faster in a direction parallel to the orientation of the long paper fibers. A typical 8.5xe2x80x3xc3x9711xe2x80x3 paper sheet has long paper fibers oriented substantially parallel to the 11xe2x80x3 direction, and moisture therefore penetrates preferentially into the 8.5xe2x80x3 edges. This causes the nominally 8.5xe2x80x3 edges to expand, so that the 8.5xe2x80x3 edges become about 1% to 2% longer than the width of the paper measured across the center of the sheet (parallel to the 11xe2x80x3 direction). It is usual practice to feed such paper sheets into a fuser nip with the 8.5xe2x80x3 edges parallel to the feeding direction, i.e., perpendicular to the roller axes. Therefore, unless corrective measures are taken, it typically takes a longer time for the swollen 8.5xe2x80x3 edges to pass through the fusing nip than it does for the middle of the sheet, which can result in severe paper wrinkling and large scale image defects. In order to provide a correction for this problem, it is known that elastomerically coated fusing station rollers can be manufactured with an axially varying profile obtained by gradually varying the thickness of the elastomeric coating, such that the outer diameter of a roller is greater near the ends of the roller than half way along the length of the roller. Inasmuch as elastomerically induced overdrive increases with increasing engagement, the larger engagements nearer the ends of the roller produce locally larger surface velocities of the paper through the nip, thereby tending to compensate for humidity induced paper swelling by having all portions of the paper spend substantially the same time passing through the nip. As is also well known, a pressure nip formed between two rollers, at least one of which has an elastomeric coating, does not usually have a uniform pressure distribution measured in the axial direction along the length of the rollers. Rather, owing to the fact that the compressive forces are applied at the ends of the rollers, e.g., to the roller axle, the rollers tend to bow outwards slightly, thereby producing a higher pressure near the ends of the rollers than half way along their length. This also tends to produce greater overdrive towards the ends of the rollers. However, the amount of extra overdrive from roller bending is not normally sufficient to compensate for humidity-induced paper swelling, and therefore a profiling of the thickness of the elastomeric coating in the axial direction, as described above, is often practiced.
As previously mentioned, PDMS cushion layers can include fillers including inorganic particulate materials, for example, metals, metal oxides, metal hydroxides, metal salts, and mixtures thereof. For example, Fitzgerald U.S. Pat. No. 5,292,606, the disclosure of which is incorporated herein by reference, describes fuser roller base cushion layers that contain fillers including particulate zinc oxide and zinc oxide-aluminum oxide mixtures. Similarly, Fitzgerald U.S. Pat. No. 5,336,539, the disclosure of which is incorporated herein by reference, describes a fuser roller cushion layer containing dispersed nickel oxide particles. Also, the fuser roller described in Fitzgerald et al. U.S. Pat. No. 5,480,724, the disclosure of which is incorporated herein by reference, includes a base cushion layer containing 20 to 40 volume percent of dispersed tin oxide particles.
Filler particles can also be included in a barrier layer. For example, in Chen et al., U.S. Pat. No. 5,464,698, the disclosure of which is incorporated herein by reference, is described a toner fuser member having a silicone rubber cushion layer and an overlying layer of a cured fluorocarbon polymer in which is dispersed a filler including a particulate mixture that includes tin oxide.
Chen et al., in U.S. patent application Ser. No. 08/879,896, disclose an improved fuser roller including three concentric layers each including a particulate filler, i.e., a base cushion layer including a condensation-cured PDMS, a barrier layer covering the base cushion and having a cured fluorocarbon polymer, and an outer surface layer including an addition-cured PDMS, the particulate fillers in each layer including one or more of aluminum oxide, iron oxide, calcium oxide, magnesium oxide, tin oxide, and zinc oxide. The barrier layer, which can include a Viton(trademark) elastomer (sold by DuPont) or a Fluorel(trademark) elastomer (sold by Minnesota Mining and Manufacturing), is a relatively low modulus material typically having a Young""s modulus less than about 10 MPa, and it therefore has a negligible effect upon the mechanical characteristics of the roller, including overdrive.
Vrotacoe et al., in U.S. Pat. No. 5,553,541, disclose a printing blanket, for use in an offset printing press, which includes a seamless tubular elastic layer including compressible microspheres, surrounded by a seamless tubular layer made of a circumferentially inextensible material, and a seamless tubular printing layer over the inextensible layer. It is disclosed that provision of the inextensible layer reduces or eliminates pre-nip and post-nip bulging of the roller when printing an ink image on a receiver sheet, thereby improving image quality by reducing or eliminating ink smearing caused by slippage associated with the formation of bulges in the prior art.
To improve image quality, and also to reduce wear and aging and thereby prolong the life of a compliant roller in a fusing station, there remains a need for a compliant fusing roller or pressure roller for use in electrostatography having a reduced or negligible propensity to exhibit overdrive behavior when engaged in a fusing nip with a non-compliant roller, or with another compliant roller. There particularly remains a need for an internally-heated compliant toner fuser roller that has a negligible propensity to produce overdrive-induced image defects, either large-scale or small-scale, when used with a relatively non-compliant pressure roller. Moreover, there is also a need for such an overdrive-controlling fuser roller to be able to provide an axially varying differential overdrive, in order to compensate for a humidity induced nonuniform swelling of receivers. The fusing station rollers of the present invention, which include a thin, flexible stiffening layer, meet these needs.
The invention provides an improved fusing station of an electrostatographic machine using an internally heated fuser roller. The fusing station includes a conformable or compliant multilayer roller, which includes a high modulus stiffening layer located near or at the surface of the roller and a preferably substantially incompressible blanket layer. The multilayer roller can include an internally heated fuser roller, or a pressure roller. The stiffening layer provides improved image quality resulting from a dramatically reduced propensity for overdrive in a fusing nip. Because of the reduced overdrive, a roller of the invention wears much more slowly and has longer operational life than a prior art roller having no stiffening layer. Preferably, the stiffening layer of an internally heated fuser roller according to the invention includes a thin high-modulus material having good thermal conductance so as to provide the roller with a more uniform surface temperature, and hence an improved fusing uniformity. An improved fusing station of the invention can include an internally heated compliant fuser roller having a stiffening layer and a compliant pressure roller having a stiffening layer, or it can include an internally heated compliant fuser roller having a stiffening layer and a hard pressure roller. Also, an internally heated hard fuser roller can be used with a compliant pressure roller having a stiffening layer. A multilayer roller having a stiffening layer can be used in simplex and duplex fusing stations. In a duplex station, each of the rollers including the fusing nip is internally heated and can have a stiffening layer.
In accordance with the invention there is provided a product and process for forming an internally heated roller configuration for use in an electrostatographic machine the employs a fuser roller and a pressure roller. One of the rollers is a conformable roller including a rigid cylindrical core member centered on an axis of rotation, a compliant base cushion layer formed on the core member; a stiffening layer in intimate contact with and surrounding the base cushion layer; and an internal heating mechanism, while the other roller is a hard roller.